Linear color space support for opengl targets

When implementing the libvlc video output callbacks for opengl targets, it seems linear color space is not supported out of the box.

assert(render_cfg.opengl_format == GL_RGBA);
assert(render_cfg.full_range == true);
assert(render_cfg.colorspace == libvlc_video_colorspace_BT709);
assert(render_cfg.primaries  == libvlc_video_primaries_BT709);
assert(render_cfg.transfer   == libvlc_video_transfer_func_SRGB);

This would be useful when integrating VLC in game engines such as Unity, where the user can choose between Gamma and Linear color spaces. Feature parity is also an argument for this, as the D3D11 output supports linear code space.